The embodiments described herein generally relate to the field of CT image reconstruction, and in particular to an apparatus and method for image reconstruction and CT system.
It is well-known that CT systems are having an increasing effect on medical diagnosis, because they can clearly image various organs of a patient so as to enable the doctors to easily identify the diseased region and to take corresponding measures of treatment. The assistance of CT systems in medical treatment has become a very important as well as an essential part in modern medicine.
Typically, a CT system includes a tube and a detector. The tube is used for emitting X-rays, and the detector is used for receiving the X-rays emitted by the tube and converting them into electrical signals to form projection data. Then, images of the irradiated subject (patient) are reconstructed according to the projection data.
The detector usually includes a symmetric detector and an asymmetric detector. The symmetric detector refers to a detector in which the numbers of detecting units at both sides of the central channel (a channel corresponding to the center of rotation) of the detector are equal or are different by no more than two channels. The asymmetric detector refers to a detector in which the numbers of detecting units at both sides of the central channel of the detector are unequal and are different by more than two channels.
The advent of the asymmetric detector in a CT system can either reduce the number of detecting units at one side of the central channel of the detector so as to reduce the cost, or increase the number of detecting units at one side of the central channel so as to enlarge the scan focus of view.
Compared to the symmetric detector, the projection data obtained by using the asymmetric detector have data loss at one side of the central channel, so the reconstruction method used for the projection data obtained by the symmetric detector is not applicable to the projection data obtained by the asymmetric detector. The reconstruction method for the projection data obtained by the asymmetric detector has, therefore, become one of the hotspots that are studied in the industry.
An article titled “X-ray micro-CT with a displaced detector array” by GE WANG published in Medical Physics, 29 (7): 1634-6 in June 2002 describes a method of reconstruction under an asymmetric detector in an axial scan. However, the reconstructed image cannot have an image quality similar to the original one in the central symmetric region under the asymmetric detector.
U.S. Pat. No. 6,873,676 and U.S. Pat. No. 6,452,996 disclose “convolution reconstruction algorithm for multi-slice CT” and “methods and apparatus utilizing generalized helical interpolation algorithm”. These patents describe the convolution reconstruction algorithm/the generalized helical interpolation algorithm, which can better suppress noises and image artifacts as compared to other two-dimensional helical reconstruction algorithms, especially for the multi-row CT having more than four rows.
U.S. Pat. No. 7,062,009 describes dividing the projection data into a symmetric region and an asymmetric region. For projection data in the symmetric region, the helical interpolation weight adopts the complementary interpolation weight, while for the projection data in the asymmetric region, the helical interpolation weight adopts the direct interpolation weight. This patent describes the reconstruction method of a helical scan under the asymmetric detector in general terms, but the reconstructed image has a quality defect. Moreover, this patent fails to provide a solution to image reconstruction for multi-row CT having more than four rows.